1. Field of the Invention
The present invention relates to pressure sensors, and more particularly to pressure sensors which compensate for temperature variations and which can provide absolute temperature readings for use, inter alia, in biomedical applications.
2. Description of the Prior Art
In the field of physiological pressure measurement transducers and particularly as to miniature intra corporeal blood pressure measuring transducers, there presently are two kinds of transducers used for internal blood measurements in biological bodies. The most commonly used is the extra corporeal type wherein a lumen in a physiological catheter conducts pressure fluctuations from a site of interest inside the biological body to a transducer located outside the biological body. An inherent disadvantage of such a system is the possibility of blood clots forming in the catheter, which, if released in the body, could harm the subject. Also, any clot formation tends to occlude the catheter lumen resulting in dampened and distorted pressure waves which lead to erroneous or difficult to interpret signals from the transducer. These potential problems are usually controlled by intermittently or continuously flushing the catheter with an anti-coagulant fluid. The flushing itself can cause problems by dislodging and discharging into the biological body an already formed clot. Additionally, a bolus of air could accidently be introduced with the flushing liquid which could have a detrimental effect on the biological body. Further, the amount of flushing permissible is limited by the biological body's tolerance to the flushing liquid. Other disadvantages of external transducer systems are the possibility of erroneous blood pressure measurements caused by a difference in elevation of the external sensor and the site of pressure measurement in the body and by artifacts superimposed on the signal of interest caused by movement of the catheter, sometimes known as catheter whip, distortion of the lumen of the catheter and by movement of the subject.
Another type of commonly used internal blood pressure measuring transducer is mounted in the catheter itself, usually at the distal end thereof. The catheter is inserted into the subject and is positioned at a site of interest. Since there is no liquid filled column, the aforedescribed disadvantages and potential dangers of an external transducer system are eliminated. Further, the superior quality of signals from an invasive transducer permits very accurate determinations of the location and extent of such problems as heart valve malformations and malfunctions. The major disadvantages of presently available invasive pressure measurement transducers are their high cost of manufacture and relative fragility.
Examples of such presently known biological transducers can be found in U.S. Pat. No. 3,550,583 to Chiku, U.S. Pat. No. 3,088,323 to Welkowitz, U.S. Pat. No. 4,342,231 to Yamamoto, U.S. Pat. No. 3,710,781 to Huthcins, U.S. Pat. Nos. 3,724,274 and 3,748,623 to Millar, U.S. Pat. No. 4,023,562 to Hynecek, U.S. Pat. No. 4,191,193 to Seo, and U.S. Pat. No. 4,274,423 to Mizuno. Several of these patents show use of temperature compensation features so that the ultimate pressure readings are not subjected to inaccuracies because of temperature variations. However, none show or suggest the employment of the elements thereof to selectively make not only pressure compensated temperature readings but also biological temperatures in the near vicinity of the transducer which are highly desirable in certain medical circumstances.
The present invention overcomes the shortcomings of the prior art by providing a relatively inexpensive and reliable transducer capable of miniaturization for use in biomedical applications which also permits measurement of biological temperatures as well as pressures compensated for temperature variations.
Outside the biomedical arts U.S. Pat. No. 3,968,466 to Nakamura, U.S. Pat. No. 4,085,620 to Tanaka, U.S. Pat. No. 4,222,277 to Kurtz, and U.S. Pat. No. 4,320,664 to Rhen show the use of crystals with multiple strain sensors or pick-ups. However, none of the references show or suggest the use of their crystals to measure biological temperature in addition to pressure and none show or suggest the use of a U-shaped crystal configuration as is taught by the invention and as will be fully described.
In numerous prior art devices separate piezoresistive crystals are employed to provide temperature compensated pressure readings. This requires handling of more than one crystal and exacting mounting, both difficult in extremely small devices. Also, the crystals must be closely matched as to their resistive characteristics to assure accuracy. The present invention overcomes these problems by employing a U-shaped crystal that is configured to provide two inherently matched sensors.
U-shaped crystals are presently used as single sensor elements when a particular length crystal is desired and would be too long for a chosen enclosure. The U-shaped shape is exploited to get the desired length in a minimum space but such crystal configurations are used as a single sensing element in contrast to the present invention wherein a single U-shaped crystal is employed to provide dual and independent sensing elements.
The use of U-shaped crystals is also known in piezo vibrator applications such as in U.S. Pat. No. 2,081,405 to Mason and U.S. Pat. No. 3,683,213 to Staudte. Further, U.S. Pat. No. 3,745,385 to Nakajima, U.S. Pat. No. 4,166,269 to Stephens, and U.S. Pat. No. 4,327,350 to Erichsen show use of notched piezo crystals either for use with conventional sensors or in microresonator applications as in Staudte and Mason.
Means for effecting transmission of forces to crystals are shown in U.S. Pat. No. 3,245,264 to Kaplan and U.S. Pat. No. 4,357,834 to Kimura.